In receivers which have to be tuned over a wide frequency range, such as e.g. the radio-frequency receivers in radio or television tuners, the reception range is divided into a plurality of subranges.
Present-day customary television receivers have to be able to receive signals in a range from 48 MHz to 860 MHz. The properties of components of the kind usually used in receivers lead to a limitation of the tuning range that can be achieved. The entire frequency range that can be achieved by a television receiver is therefore divided e.g. into three ranges. In this case, a first range extends from 48 to 150 MHz, a second range extends from 150 to 430 MHz, and a third range extends from 430 to 860 MHz. The tuning over the three ranges is carried out by means of a three-band or a switched two-band concept in modern receivers.
In the case of the three-band concept shown in FIG. 1, the signal received by an antenna is conducted onto three parallel branches B1, B2 and B3. Each branch is designed for a frequency range and comprises an input filter EF1, EF2 and EF3, an input amplifier A1, A2 and A3, as well as a bandpass filter BF1, BF2 and BF3. The output signals of each of these branches are passed to a respective mixer MI1, MI2, and MI3, which is assigned a respective oscillator O1, O2 and O3. An intermediate frequency can be tapped off in each case at the output of the mixers.
In order to reduce the circuitry outlay, a three-band concept in which there are only two mixers and oscillators is also employed. In the case of this concept shown in FIG. 2, one frequency range, e.g. B3, is connected to a mixer MI3 and an oscillator O3 as in the three-band concept described above. The two remaining frequency ranges B1 and B2 are firstly conducted, as in the three-band concept described above, via a respective individual branch with input filters EF1 and EF2, input amplifiers A1 and A2 and bandpass filters BF1 and BF2, wherein at the output of the bandpass filters a switch S feeds in each case one of the two branches to a common mixer MI1 and oscillator O1.
A further reduction of the circuitry outlay can be achieved by using a switched two-band concept as shown in a diagrammatic illustration in FIG. 3. As in the two concepts described above, the switched two-band concept has an individual branch comprising input filter EF3, input amplifier A3 and bandpass filter BF3 for one frequency range, e.g. B3, which is passed to a dedicated mixer MI3 and oscillator O3. The two remaining frequency ranges B1 and B2 are passed via a common branch comprising input filter EF1/2, input amplifier A1 and bandpass filter BF1/2 and fed to a dedicated mixer MI1 and oscillator O1. The properties of the input filter EF1/2 and the bandpass filter BF1/2 are switchable in the case of the switched two-band concept. As a result, the filters can be adapted to the frequency range respectively selected.
The switchable bandpass filter is embodied as an inductively reference-point-coupled two-circuit bandpass filter and is illustrated in a simplified embodiment in FIG. 4. A signal S passes from an antenna (not illustrated in FIG. 4) and an input filter (likewise not illustrated) via the input amplifier A1 to a first primary resonant circuit. The first primary resonant circuit comprises the variable capacitance C1 and the series circuit formed by the inductances L1 and L2. The switches S1 and S2 are open in this case. One connection of the inductance L2 is connected to a first connection of an inductance L3 and completes the series circuit of the first primary resonant circuit. The second connection of the inductance L3 is at a fixed potential. A first secondary resonant circuit comprising the series circuit formed by the inductances L4 and L6 as well as a variable capacitance C2 is likewise connected to the first connection of the inductance L3, so that a T-shaped arrangement is produced overall. The coupling elements RK and CK are connected to the output-side end of the bandpass filter, which coupling elements connect the bandpass filter BF1/2 to the mixer MI1. In order to switch the frequency range of the bandpass filter BF1/2, the two switches S1 and S2 may be closed. The inductances L2 and L4 are thus short-circuited and the circuit has a new common reference point inductance L5. In this case, the inductance L5 is expediently chosen to be smaller than the inductances L2 and L4, so that the components connected to these elements can subsequently be disregarded. In one exemplary embodiment, the values of the inductances differ by the factor three. The second primary resonant circuit thus produced then comprises the variable capacitance C1 and also the series circuit formed by the inductances L1 and L5. The second secondary resonant circuit comprises the series circuit formed by the inductances L6 and L5 and also the variable capacitance C2. The bandpass filter that is frequency-switched in this way furthermore remains connected to the coupling elements RK and CK and consequently connected to the mixer. The advantage of the lowest outlay compared with the first two concepts is opposed by the disadvantage that the coupling elements CK and RK can be set optimally only for one range. As a result, relatively large level differences can occur over the two frequency ranges and impair the properties of a tuner.
In order to avoid this effect, although it is possible for the coupling elements to be configured in switchable fashion, this nevertheless results in an increase in the circuitry outlay again.
For these reasons there is a need for a switchable tuneable bandpass filter which has a uniform level profile over the two frequency ranges with a low circuitry outlay.